Air conditioning system having supercooled phase change material

ABSTRACT

An air conditioning system includes a chiller system including a compressor, a condenser, an expansion device and an evaporator; a phase change material in thermal communication with the condenser; an actuator coupled to the phase change material; and a controller providing a trigger signal to the actuator to initiate changing the phase change material from a supercooled state to a solid state.

BACKGROUND

The subject matter disclosed herein generally relates to airconditioning systems, and in particular relates to an air conditioningsystem utilizing supercooled phase chase material to store thermalenergy.

Existing air conditioning systems employ phase change materials toimprove capacity and/or efficiency of the system. Exemplary airconditioning systems include energy storage systems which freeze a phasechange material when energy costs are relatively low (e.g., non-peakrates). The phase change material is then used to absorb thermal energyduring other modes of operation to improve efficiency and/or capacity ofthe air conditioning system.

SUMMARY

One embodiment is an air conditioning system which includes a chillersystem including a compressor, a condenser, an expansion device and anevaporator; a phase change material in thermal communication with thecondenser; an actuator coupled to the phase change material; and acontroller providing a trigger signal to the actuator to initiatechanging the phase change material from a supercooled state to a solidstate.

Another exemplary embodiment is a method for operating an airconditioning system having a chiller system including a compressor, acondenser, an expansion device and an evaporator, a phase changematerial in thermal communication with the condenser, and an actuatorcoupled to the phase change material, the method including determiningwhether an ambient temperature profile will result in supercooling ofthe phase change material; and in response to the determining,triggering the actuator to initiate changing the phase change materialfrom a supercooled state to a solid state.

Other exemplary embodiments and features are described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts an air conditioning system in an exemplary embodiment.

FIG. 2 depicts a condenser coil assembly in an exemplary embodiment.

FIG. 3 depicts an actuator in an exemplary embodiment.

FIG. 4 is an plot of temperature versus time, illustrating phase changematerial state changes in an exemplary embodiment.

FIG. 5 is a flowchart of a control process in an exemplary embodiment.

DETAILED DESCRIPTION

FIG. 1 depicts an air conditioning system in an exemplary embodiment. Achiller system includes a compressor 10, a first heat exchanger 12, anexpansion device 14 and a second heat exchanger 16. The first heatexchanger 12 may be used as a condenser coil, and may be located outsideof a building or space to be conditioned. The second heat exchanger 16may be used as an evaporator coil. As known in the art, refrigerant issubjected to a vapor compression cycle through compressor 10, condenser12, expansion device 14 and evaporator 16. Heat is absorbed atevaporator 16 and heat is discharged at condenser 12.

The system of FIG. 1 may be a water chiller system. Evaporator 16 is inthermal communication with a heat exchanger 18 (e.g., a coil) thatcarries a fluid coolant, e.g., water. A supply pump 20 circulatescoolant from heat exchanger 18 cooled by evaporator 16 to a supply valve22. Supply valve 22 supplies chilled water (about 45° F.) to a localzone terminal where a fan draws air over a coil to chill a space asknown in the art. A return valve 24 receives fluid returned from thelocal zone terminal and provides the return fluid to heat exchanger 18.It is understood that embodiments of the invention may be used withother types of air conditioning systems (e.g., forced air) andembodiments are not limited to water chiller systems.

Condenser coil 12 is in thermal communication with a phase changematerial 26. Condenser coil 12 may be fully embedded in the phase changematerial 26 or the phase change material may be in a housing containingcondenser coil 12. Alternatively, a portion of the condenser coil may beexposed to ambient air. A fan 28 may draw air through the phase changematerial 26 to aid in cooling the phase change material 26. In exemplaryembodiments, phase change material 26 is a material that achieves asupercooling state. A controller 32 then initiates the transition of thephase change material 26 from supercooled liquid to solid. An actuator30 is used to initiate the transition of the phase change material 26from supercooled liquid to solid when the phase change material 26 is ina supercooled state, as described in further detail herein.

A controller 32 controls operation of the system. Controller 32 may beimplemented using a general purpose microprocessor executing computercode stored in a storage medium for performing the functions describedherein. Controller 32 receives a phase change material temperaturesignal from a phase change material sensor 34 in thermal contact withphase change material 26. Controller 32 also receives an ambienttemperature signal from an ambient temperature sensor 36. Ambienttemperature sensor 36 may monitor the outside air temperature in thevicinity of condenser 12. Controller 32 may send control signals tocompressor 10, pump 20, supply valve 22, return valve 24, fan 28 andactuator 30. Operation of the system is described in further detailherein with reference to FIG. 5.

FIG. 2 depicts a condenser assembly in an exemplary embodiment.Condenser coil 12 is in thermal communication with phase change material26. In the embodiment of FIG. 2, the condenser coil 12 is embedded inphase change material 26. In other embodiments, a portion of condensercoil 12 may extend beyond the phase change material 26, such that theentire condenser coil 12 is not embedded in the phase change material26. Air paths 40 are formed in the phase change material 26 to allow airto be drawn through the phase change material 26 by fan 28. The airpaths may be arranged in a variety of configurations, and embodimentsare not limited to the arrangement shown in FIG. 2. Controller 32 mayturn fan 28 on when additional ambient airflow is needed to cool thephase change material 26.

A coolant supply line 42 is also in thermal communication with the phasechange material 26, and may be embedded in the phase change material 26as shown in FIG. 2. In situations where the ambient temperature isinsufficient to adequately cool the phase change material 26 (e.g., to asupercooled state), controller 32 may direct chilled coolant from supplyvalve 22 to the phase change material 26 and back to return valve 24.Controller 32 activates compressor 10 to produce chilled coolant in coil18. Controller 32 sets valves 22 and 24 to route chilled coolant to thephase change material 26 and pump 20 is activated circulate the chilledcoolant to the phase change material 26.

FIG. 3 depicts an actuator 30 in an exemplary embodiment. Actuator 30 iscontrolled by controller 32 to initiate a transition from supercooled,liquid phase change material to solid phase change material.Supercooled, as used herein, refers to the phase change material 26being in a liquid state and at a temperature below the phase changematerial freezing temperature. The actuator in FIG. 3 includes a tube 50of phase change material and a thermoelectric cooler 52 that maintainsthe phase change material in tube 50 in a frozen or solid state underall conditions. Valve 54 connects the tube 50 to the main reservoir ofphase change material 26. Controller 32 opens valve 54 to triggerfreezing of the main phase change material reservoir 26. When valve 54is opened, some of the unfrozen liquid in phase change material 26 flowstoward the frozen volume in tube 50 and begins to freeze. The freezefront moves outward from the valve area to the rest of the phase changematerial 26 as the latent heat is released. Another embodiment ofactuator 30 includes an ultrasonic emitter to produce ultrasonic soundwaves to trigger transition of the phase change material 26 fromsupercooled liquid to solid.

As described in further detail herein, the phase change material 26 isselected so that the phase change material transitions from liquid tosolid when cooling demand on the chiller system is low or non-existent.This may occur in the evening, when ambient temperatures are lower. FIG.4 illustrates an exemplary diurnal temperature versus time profile,along with states of the phase change material 26. In the example ofFIG. 4, the outside air temperature ranges from about a 95° F. day timehigh to about a 70° F. night time low. If the transition temperature ofthe phase change material 26 is selected to be about 75° F., then thephase change material 26 will be frozen (or recharged) at night and thenmelt (or discharge) during the day. During the day, the frozen phasechange material 26 absorbs energy from the condenser coil 12, improvingthe efficiency of condenser 12 when the chiller system is running andincreasing efficiency and capacity of the chiller system.

FIG. 5 is a flowchart of a control process executed by controller 32 inan exemplary embodiment. The process begins at 100 where the controller32 obtains climate zone data for the system. The climate zone data maybe indicated by known climate maps and programmed into controller 32upon installation of the air conditioning system. At 102, controller 32monitors the ambient air temperature through ambient temperature sensor36 and may store multiple ambient temperatures over time. At 104controller 32 obtains the phase change material temperature from phasechange material temperature sensor 34, and may store multiple phasechange material temperatures over time.

At 106, controller 32 predicts a nighttime temperature profile based onthe climate zone data and one or more ambient air temperature readingsover time. Controller 32 may be preloaded with predicted nighttimetemperature profiles indexed by climate zone data and daytime ambientair temperatures.

At 108, controller 32 determines if the predicted nighttime temperatureprofile will be sufficient to supercool the phase change material 26,based on one or more phase change material temperature measurements. Forexample, if the phase change material 26 transition temperature is about75° F., the current phase change material temperature is about 80° F.and the predicted nighttime temperature profile indicates four hours ofambient air temperature of about 72° F., then controller 32 maydetermine that the predicted nighttime temperature profile will resultin the phase change material supercooling before the next chiller cycleis initiated (i.e., before the space being conditioned requirescooling). This determination will be affected by factors such as theamount of phase change material, its temperature transitioncharacteristics, etc.

If the ambient temperature alone is sufficient to supercool the phasechange material 26, flow proceeds to 110 where the controller determinesa trigger time to transition the supercooled phase change material 26from liquid to solid. Controller 32 attempts to trigger this transitionwhen the ambient temperature is at or near a minimum value, so that theheat released by the phase change material 26 is more rapidly absorbedby the ambient air. At 112, controller sends a trigger signal toactuator 30 at the trigger time to initiate transition of thesupercooled phase change material 26 to a solid.

If at 108 the predicted nighttime temperature profile is insufficient tosupercool the phase change material 26, flow proceeds to 114 where it isdetermined if running fan 28 to draw ambient air through the phasechange material 26 will result in supercooling of the phase changematerial 26. This determination may be made by controller 32determining, based on the current phase change material temperature,that only a small temperature decrease is needed to supercool the phasechange material 26. If so, fan 28 is turned on at 116 and flow proceedsto 110 and 112 as described above.

If at 114 controller 32 determines that the fan 28 will not supercoolthe phase change material, flow proceeds to 118 where controller 32 runsthe chiller system including compressor 10 and pump 20. At 120, supplyvalve 22 and return valve 24 are set by controller 32 to direct coolantfrom coil 18 to the phase change material 26. Flow proceeds to 110 and112 as described above.

Embodiments employ a phase change material that meets cost objectivesbut has a transition temperature high enough so that the nighttimetemperature drops below the transition temperature each night. A phasechange material is chosen that has a high propensity for supercooling.As the nighttime temperature drops below the starting temperature of thephase change material, sensible cooling takes place according to theheat capacity of the phase change material. When the nighttimetemperature is near a minimum, the supercooled phase change material istriggered to release its latent heat quickly.

The temperature of the phase change material rises according to the heatcapacity driven by the latent heat release until the limit of themelting temperature is reached. This provides a higher temperaturedifference between the outdoor air and the phase change material and theheat transfer from the phase change material to the outside air canoccur at faster rates. The heat exchanger design can be optimize to takeadvantage of this rapid heat release. An example of a candidate phasechange material with a transition temperature in the right region whichis known to exhibit supercooling and which may be inexpensive enough isnatural coconut fatty acid mixture.

Embodiments harness supercooling for positive uses by permitting thedaytime heat captured in a medium to be released over a shorter periodof cooler night air than otherwise would be possible. The difference intemperature created in the phase change material between the outdoor airtemperature and the melting point temperature permits faster heatrelease to the environment and downsizing of the associated heatexchanger. This increases the viability of thermal energy storage from acost/benefit perspective.

Chillers normally reject heat into hot outside air (95° F. rating T)during periods of occupancy. The “lift” from the chilled watertemperature (CWST) to the outside temperature (OAT) governs chillerefficiency, as illustrated in FIG. 4. The chiller may be off during mostof the setback period during unoccupancy.

The advent of less expensive phase change materials that have a choicetransition temperature (T_(m)) means that systems can be designed topick the better of the OAT and T_(m) to reject heat during periods ofoccupancy, lowering the chiller lift and increasing chiller efficiencywhen electric rates are typically highest. The phase change materialdischarges throughout the day. During periods of unoccupancy at nightwhen the chiller runs infrequently, the phase change material isrecharged by cooler night air after the night air temperature dropsbelow T_(m) in an “economizer” mode. Embodiments use a phase changematerial that exhibits supercooling so that when triggered, the phasechange material temperature rises relative to the night air and therecharge goes faster. If necessary, the chiller system can assist so asto complete the recharge of the phase change material before morningoccupancy. If the chiller system is needed, it will operate at lowerlift than it would have during the day and use cheaper electricity.

While the invention has been described in detail in connection with onlya limited number of embodiments, it should be readily understood thatthe invention is not limited to such disclosed embodiments. Rather, theinvention can be modified to incorporate any number of variations,alterations, substitutions or equivalent arrangements not heretoforedescribed, but which are commensurate with the spirit and scope of theinvention. Additionally, while various embodiments of the invention havebeen described, it is to be understood that aspects of the invention mayinclude only some of the described embodiments. Accordingly, theinvention is not to be seen as limited by the foregoing description, butis only limited by the scope of the appended claims.

1. An air conditioning system comprising: a chiller system including acompressor, a condenser, an expansion device and an evaporator; a phasechange material in thermal communication with the condenser; an actuatorcoupled to the phase change material; and a controller providing atrigger signal to the actuator to initiate changing the phase changematerial from a supercooled state to a solid state.
 2. The air conditionsystem of claim 1 wherein: the condenser is embedded in the phase changematerial.
 3. The air condition system of claim 1 further comprising: anambient temperature sensor providing an ambient temperature signal tothe controller.
 4. The air condition system of claim 1 furthercomprising: a phase change material sensor providing a phase changematerial temperature signal to the controller.
 5. The air conditionsystem of claim 1 wherein: the phase change material includes aninternal airway for allowing ambient air to flow through the phasechange material.
 6. The air condition system of claim 5 furthercomprising: a fan for drawing air through the airway.
 7. The aircondition system of claim 1 wherein: the phase change material includesa coolant supply line in thermal communication with the phase changematerial, the coolant supply line coupled to the chiller system.
 8. Theair condition system of claim 1 wherein: the actuator generates a soundwave to initiate changing the phase change material from the supercooledstate to the solid state.
 9. The air condition system of claim 1wherein: the actuator includes frozen phase change material, the frozenphase change material being placed in contact with the phase changematerial to initiate changing the phase change material from thesupercooled state to the solid state.
 10. The air condition system ofclaim 9 wherein: the actuator includes a valve separating the frozenphase change material from the phase change material, the controlleropening the valve to initiate changing the phase change material fromthe supercooled state to the solid state.
 11. The air condition systemof claim 1 wherein: the controller predicts a nighttime temperatureprofile in response to climate zone data and ambient temperature; andthe controller determining if the phase change material will supercoolin response to the nighttime temperature profile.
 12. The air conditionsystem of claim 11 wherein: the controller providing the trigger signalto the actuator in response to the nighttime temperature profile. 13.The air condition system of claim 11 wherein: the phase change materialincludes an internal airway for allowing ambient air to flow through thephase change material; when the controller determines that the phasechange material will not supercool in response to the nighttimetemperature profile, the controller activating a fan for drawing airthrough the airway to supercool the phase change material.
 14. The aircondition system of claim 11 wherein: the phase change material includesa coolant supply line in thermal communication with the phase changematerial, the coolant supply line coupled to the chiller system; whenthe controller determines that the phase change material will notsupercool in response to the nighttime temperature profile, thecontroller activating the chiller system to cool the coolant supply lineto supercool the phase change material.
 15. A method for operating anair conditioning system having a chiller system including a compressor,a condenser, an expansion device and an evaporator, a phase changematerial in thermal communication with the condenser, and an actuatorcoupled to the phase change material, the method comprising: determiningwhether an ambient temperature profile will result in supercooling ofthe phase change material; and in response to the determining,triggering the actuator to initiate changing the phase change materialfrom a supercooled state to a solid state.
 16. The method of claim 15wherein: triggering the actuator includes determining a trigger time inresponse to the ambient temperature profile and triggering the actuatorat the trigger time.
 17. The method of claim 15 further comprising:running a condenser fan in response to determining that the ambienttemperature profile will not result in supercooling of the phase changematerial.
 18. The method of claim 17 further comprising: running thechiller system in response to determining that the ambient temperatureprofile will not result in supercooling of the phase change material andthe running the condenser fan will not result in supercooling of thephase change material.
 19. The method of claim 18 wherein: running thechiller system includes cooling the phase change material with coolant.20. The method of claim 15 wherein: determining whether the ambienttemperature profile will result in supercooling of the phase changematerial includes predicting a nighttime temperature profile in responseto climate zone data, daytime temperature and a phase change materialtemperature.